Photomasks are used in microlithography in printing miniature circuit patterns on silicon wafers and carry an enlarged version of the circuit to be printed thereon. To reduce the size of the circuits on the silicon wafers to get more circuits on the same wafer, light with lower wavelengths are used. For laser light with a low wavelength (less than 248 nm), the photomask substrate can be made with fused silica glass that has high transmitivity. To display high transmitivity, fused silica glass needs to be very pure and contain extremely low levels of water (preferably less than about 10 ppb). The presence of large amounts of water in fused silica product makes the glass not suitable for certain low wavelength applications. Current systems operate at the 248 nm window. Lower wavelength systems heretofore have been largely unsuccessful because of water levels being too high in the silica photomask material. Thus, it would be desirable to produce a glass material that could be used at lower wavelengths.
One process that delivers glass with lower levels of water is the process used to make preforms for optical fiber waveguides (hereinafter the “preform manufacture process”). This preform manufacturing process utilizes several manufacturing steps. First silica containing soot is deposited onto, for example, an alumina bait rod by an Outside Vapor Deposition (OVD) method, for example. The bait rod is removed leaving a tubular soot member with a centerline aperture. This soot member may include the appropriate dopants, for example germania, such that a desired refractive index profile is achieved. The soot preform is then consolidated in a furnace with a vacuum generally applied to close the centerline aperture. Next, the consolidated preform is drawn into core cane; wherein the core cane preferably comprises part or all of the physical core of the optical fiber when finally drawn into fiber. This core cane is cut into lengths and again overclad with silica-containing soot to form the clad portion or another segment of the core if a multi-segment profile is desired. The preform is again consolidated. Chlorine gas, for example, in the atmosphere of the consolidation furnace is used to dry the preform and remove water prior to vitrification into glass in both the above-mentioned consolidation steps. The resulting final consolidated preform, is then placed in a draw furnace and drawn into a fiber in an inert gas atmosphere.
Unfortunately, because of the process currently used to form the soot, water is inevitably formed into the preform. Therefore, it is necessary to employ a drying step before consolidation. Specifically, the water is formed, as will be hereinafter described, because the chemical reaction of the silica precursors and fuels currently used in the process of forming soot form water as a reaction by-product. Moreover, it was discovered by the inventors herein that exposure to atmospheric conditions during standard processing techniques causes the soot preform to pick up further water. In optical telecommunications systems, one factor that determines the distance between amplification stages is the optical fiber attenuation. A significant contributor to poor attenuation is water (OH) present in the preform. Water present causes a peak in the transmission curve at about 1383 nm. This peak has a detrimental effect on the attenuation at 1550 nm, a primary transmission wavelength in optical fiber communications. Thus, it is desirable to reduce the water peak by reducing the water content of the consolidated glass as much as possible.
Furthermore, in fluorine doped optical fibers, fluorine doping at acceptable levels is a considerable problem. Moreover, once fluorine is present in the soot preform, fluorine migration is a significant problem because of fluorine's high mobility and small molecular size. Fluorine is utilized as a refractive index depressant, thus desirably enabling negative indices of refraction where desired. Migration dramatically reduces the amount of fluorine that may be incorporated in the soot. Moreover, migration smoothes out the refractive index profiles desired for optimal signal transmission. Thus, rather than achieving sharp transitions between profile regions, migration causes rounded transitions. Moreover, migration lowers the delta % value (a measure of the refractive index difference relative to the cladding). Thus, since fluorine is extremely mobile, it is very desirable to achieve a method and/or apparatus to prevent migration of such dopants throughout the soot preform during processing.
Equation 3 illustrates the forming high purity fused silica or silica soot in accordance one process used in the prior art. SiCl4 (a silica precursor), oxygen and methane are combined and ignited in a burner to produce glass or soot which is deposited on a substrate surface. In the case of high purity fused silica, the soot is substantially simultaneously consolidated (vitrified) within the furnace when methane is utilized. The by-products of such reactions are carbon dioxide, water vapor and chlorine. In particular, large amounts of water vapor are produced.CH4+3O2+SiCl4→CO2+2H2O+SiO2+2Cl2  (Prior Art 1)
Another currently employed process for manufacture of silica soot uses octamethyl-cyclo-tetra-siloxane (OMCTS) as the raw material for silica soot and natural gas (predominantly methane along with other hydrocarbons) as the fuel. Natural gas is utilized as the fuel to maintain the furnace at high temperatures for manufacture of high purity fused silica. The products of combustion of the natural gas are also water vapor and carbon dioxide. The products of combustion of the OMCTS are silica, water and carbon dioxide as shown in equation 2.C8H24O8Si8+16O2→8CO2+12H2O+8SiO2  (Prior Art 2)
Thus, it should be recognized that a significant by-product of the reaction in both processes outlined in equations 1 and 2 is water vapor generated as a result of combustion. Undesirably, this water gets incorporated in soot, and, once present, is very difficult to remove. To attempt to remove the water from soot articles, such as soot preforms, extensive drying step utilizing chlorine are employed. Detrimentally, however, some water remains captured in the consolidated glass produced. The presence of water is detrimental to optical properties of the glass produced. Thus, it is an industry-wide goal to further reduce the water content present in high purity fused silica and also in silica-soot articles such as soot preforms for optical fiber manufacture.